Water-based inks, which contain water as the main solvent, are advantageously free from or have a lower level of fire risk and toxicity, such as mutagenicity, of solvent inks. Thus, water-based inks are the mainstream of ink jet recording inks except for industrial applications.
The characteristics required for ink jet recording inks include (1) formation of uniform images having good color development, high resolution, and a high concentration without blurring on recording media, (2) high ejection stability without clogging of the nozzle tip due to drying of ink, (3) good drying characteristics of inks on recording media, (4) good image fastness, and (5) high long-term storage stability.
In water-based inks for ink jet recording, dyes are used as colorants because of their high dissolution stability, little nozzle clogging, high color developability, and capability of printing high-quality images. However, images formed of dyes have problems of low water fastness and lightfastness.
In order to solve these problems, attempts to convert the colorant from dyes to pigments have been actively made. Although pigment inks are expected to have good water fastness and lightfastness, pigment inks have lower color developability than dye inks and have a nozzle clogging problem due to aggregation and sedimentation of pigment. Thus, methods for dispersing a fine pigment in an aqueous medium using a polymer dispersant and the dispersion methods themselves have been studied.
In order to produce pigment inks having good ink jet characteristics without nozzle clogging, it is necessary to disperse a particulate pigment in an aqueous medium so as to achieve the desired characteristics. However, even when a pigment dispersion liquid is produced by dispersing a pigment, the pigment dispersion liquid cannot function as an ink jet recording ink until the surface tension and viscosity of the pigment dispersion liquid are adjusted for various ejection systems exemplified by a piezoelectric system and a thermal system.
In the case of dye inks, ink jet recording inks can be produced by only adding a dye to an aqueous medium containing no color material. In the case of pigment inks, ink jet recording inks are generally produced by two steps in this technical field: a step of optimally dispersing a pigment in an aqueous medium and a step of preparing an ink suitable for ink jet recording from the resulting aqueous pigment dispersion liquid. Thus, the problem of aggregation and sedimentation of pigment must be solved in both aqueous pigment dispersion liquids and ink jet recording inks.
When a color material is used as a pigment in an aqueous ink jet recording ink, the concentration of pigment in the ink generally ranges from 0.5% to 10% by mass, and the concentration of pigment in a pigment dispersion liquid used for the preparation of the ink generally ranges from approximately 5% to 25% by mass. It is important to prevent aggregation of pigment present in a much higher concentration in pigment dispersion liquids than in inks and to consistently maintain the dispersion state.
Dispersion of pigment is generally reversible. Thus, unless a measure to prevent reaggregation of pigment is taken, it is difficult to maintain the dispersion state of dispersed pigment. In many cases, therefore, a resin having both a group called an anchor that has a high affinity for the pigment surface and a group called a chain that has an affinity for a dispersion medium (the term “dispersion medium”, as used herein, refers to a liquid medium used for dispersion) or a binder resin is used as a dispersant, and a resin layer is formed on the surface of pigment particles to stabilize dispersion. (See, for example, Non-Patent Literature 1.)
A dispersion method including a pretreatment step of kneading a pigment and a polymer dispersant in advance is proposed as a method for forming a resin layer on the surface of pigment particles to stabilize dispersion (see, for example, Patent Literature 1). A resin layer can be more efficiently formed on the surface of pigment particles through a pretreatment step of kneading a pigment and a polymer dispersant in advance. This can form more stable dispersion, improve production efficiency, and decrease the pigment size.
Although pigment particles become finer, there is room for improvement in the storage stability of ink due to insufficient pigment dispersion stability. In particular, quinacridone pigments used for the production of magenta inks are hydrogen bond type pigments, which function as pigments via intermolecular hydrogen bonds. Thus, pigment particles aggregate strongly, and undispersed coarse particles tend to remain in the inks. Furthermore, dispersed fine pigment particles tend to reaggregate. This makes it more difficult to stabilize dispersion.
Addition of a pigment derivative as a pigment dispersant is known to solve this problem. Pigment derivatives have a chemical structure similar to that of pigments and have a group that can bind to a binder or has an affinity for a binder, such as a dialkylaminomethyl group, an arylamidemethyl group, a sulfonic acid amide group, a sulfonate group or a salt thereof, or a phthalimide group (see, for example, Patent Literature 1). It is assumed that a portion of a pigment derivative that is identical or similar in the chemical structure to a pigment is adsorbed onto the pigment and thereby improves the dispersion or dispersion stability of the pigment. For example, an aqueous pigment dispersion liquid containing a quinacridone pigment and a phthalimidemethylated quinacridone compound is proposed (see Patent Literature 2). An aqueous pigment dispersion containing a quinacridone pigment, a phthalimidemethylated quinacridone compound, and a quinacridone sulfonic acid compound is proposed (see Patent Literature 3).
However, it is sometimes difficult to disperse strongly aggregated coarse particles even using these methods.
When a cartridge is filled with an ink containing many coarse pigment particles, the coarse particles sediment with the lapse of time and are deposited in the vicinity of an ink ejection port of a nozzle. The deposition may impair ink ejection performance and cause blurring or a low print density in printed matter. Although a cartridge cleaning function, which is generally installed in printers, is used to remove the deposition, this method not only takes a lot of time and effort but also consumes a considerable amount of ink, thus increasing printing costs.